1. Field of the Invention
The utility of this invention is to provide a high capacity, efficient, inexpensive, and compact magnetic refrigerator in the temperature region of 2 K to room temperature (293 K). Magnetic refrigeration involves the expulsion of heat into a high temperature sink from the magnetic working material through the application of a magnetic field. The subsequent removal of the magnetic field cools the working material and allows absorption of heat from a low temperature bath. This process and device operates at high Carnot efficiency, requires no massive gas compressors, and is compact because solids instead of gases are used as the working material.
Using the device and method of this invention, the inventor has found that at high temperatures the lattice and electronic contribution to the working material specific heat is very large. One of the problems solved is that although the working material is able to remove a large quantity of heat from the fluid being refrigerated and to cool that fluid, it is unable to cool very far (not above a 30 K span) because of its own large specific heat. The inventor has solved this problem by forcing a cold fluid to flow in intimate contact through a permeable wheel of magnetic working material and thus cooling the wheel in a high magnetic field. As the working material rotates out of the high field region, it gets even colder. This very cold wheel then is used to cool the fluid to this very low temperature. Now the very cold fluid can absorb heat from a thermal load. In cooling this fluid, however, the wheel has warmed up in preparation for being recooled by the fluid after it enters the high field region. During the cycle heat is expelled by the fluid into the thermal reservoir. The magnetic working materials are operated near their ferromagnetic Curie temperatures; thus, their own internal spinspin coupling enhances the externally applied magnetic field.
2. Prior Art
The principle of magnetic refrigeration is very old and, simply stated, the principle is that the application of magnetic field to a material warms the material and expels heat from the material into a high temperature thermal reservoir. The subsequent removal of the magnetic field causes the material to cool and absorb heat from the substance to be refrigerated. The following is a list and abstracts of the most closely related art known to applicant:
(1). U.S. Pat. No. 2,510,800, C. Chilowsky, is directed to the transformation of thermal energy into mechanical or electrical energy using paramagnetic bodies or ferromagnetic bodies and, using the Curie point of the materials, to effect the change from thermal to electrical energy. In addition, this patent suggests the use of liquid metals. A distinction between the apparatus and method of the patent over the present invention is that the invention of the patent does not expose the stationary working material to a varying magnetic field. In this invention the rotating working material is exposed to both very large and zero magnetic fields during one cycle. The Chilowsky patent method is inefficient since the magnetization of a ferromagnet does not change sharply with temperature in an applied field, even near its Curie point.
(2). U.S. Pat. No. 2,589,775, C. Chilowsky. The method of refrigeration in an apparatus containing ferromagnetic sections having Curie point temperatures approximating the desired temperature of refrigeration located in gaps in a closed ferromagnetic armature, which comprises causing a magnetic flux to traverse the armature, subjecting the ferromagnetic sections alternately to magnetization and demagnetization, passing a fluid in heat-exchange relation with said sections alternately in opposite directions, such passage of fluid being so timed that the phase of magnetization of each section coincides with the passage of fluid in one direction and the phase of demagnetization coincides with passage in the opposite direction, removing heat from the fluid after passing a section in the former direction, and supplying heat to the fluid after passing a section in the latter direction, whereby the space from which heat is supplied is refrigerated. In particular, U.S. Pat. No. 2,589,775 differs in the following respects from the device and method of this invention in that the present invention uses rare earths instead of 3d elements (periodic table) because of their much larger magnetic moment and a superconducting magnet with an intense and high field rather than an iron magnet. The primary distinction is that in this invention the material to be cooled is rotated in and out of the magnetic field, whereas in U.S. Pat. No. 2,589,775 the working material is stationary.
(3). U.S. Pat. No. 3,841,107, Arthur C. Clark. A magnetic refrigeration system includes thermal transfer means comprising a serial arrangement of magnetocaloric elements and a source of magnetic field. The serial arrangement comprises a material having a large, negative magnetocaloric effect which cools upon application of a magnetic field; a paramagnetic material in abutting relationship therewith which cools upon removal of a magnetic field; and end elements functioning as thermal switches. The magnetic field is caused to move along the serial arrangement, permitting heat to be transferred from a heat source to a heat sink. Cascading of the serial arrangements increases the refrigeration effect.
A distinction of the cited patent over the present invention is that U.S. Pat. 3,841,107 uses magnetic switches and therefore is useful only as a very low power refrigerator since metals carry heat poorly compared to the forced flow of the present invention. In addition, the present invention involves rotating the magnetic material, thus providing very rapid cycle rates.
(4). U.S. Pat. No. 3,108,444, D. Kahn. A magnetocaloric cryogenic refrigerator comprising: a pair of spaced, thermally isolated heat reservoirs, a material having superconducting properties thermally connecting said reservoirs with said material being the sole thermal connecting means between said reservoirs, means for subjecting said material to a temperature sufficiently low to cause superconductivity therein, means for subjecting only a portion of said material to a magnetic field of critical field intensity to cause said subjected portion, while thermally isolated, to revert to its normal state with a subsequent decrease in temperature and means for effecting progressive relative movement between said material and said magnetic field to cause a net heat transfer from one reservoir to the other.
The cited patent use superconductors while the apparatus of this invention uses paramagnets and ferromagnets. The Kahn patent separates the field from the fluid by a membrane and thus has poor contact between the working material and the fluid. The present invention forces fluid through permeable magnetic working material.
(5). U.S. Pat. No. 3,393,526, J. Pearl. Heat is pumped from one chamber, which is below the critical temperature of a superconductive material, into another chamber, which is also below the said critical temperature, by placing the ends of a rod or rods of that material in heat transfer relation to the two chambers respectively and by applying a magnetic field, which is strong enough to cause a zone of said rod or rods to become normal, to the end of the rod or rods that is in heat transfer relation with the first chamber. When the zone on the rods becomes normal, it withdraws heat from the first chamber, cooling it. Then the magnetic field, and therefore the normal zone, is moved along the rod to the second chamber, whereby the second chamber absorbs the heat that is trapped in the normal zone and that moves with it. The process may be repeated to still further cool the first chamber. The Pearl patent differs from the high temperature refrigerator of this invention in the following ways: it uses metal into a forced mass transport to carry the heat and uses a superconductor instead of a rotating para- or ferromagnet.
(6). U.S. Pat. No. 3,413,814, J. R. Van Geuns. A method and apparatus for producing cold in which the entropy of a paramagnetic substance is alternately varied by varying an external parameter such as a magnetic field, and a fluid medium such as helium gas is flowed in alternate directions in heat-exchange relationship with the substance. During the directional flows heat and cold, respectively, are dissipated from the substance to the fluid, and corresponding to these flows there is heat-exchange relationship first by a portion of the fluid with an area absorbing heat from the fluid, and subsequently, by a remote portion of the fluid with an area to be cooled. Fluid in the first area is at a generally higher temperature than fluid in the area to be cooled, and portions of fluid in the two areas are not intermixed.
The Van Geuns patent uses nonrotating nonferromagnets while this invention is directed to rotating para- or ferromagnets since cooling is to be done above 20 K which requires the use of ferromagnets. U.S. Pat. No. 3,413,814 teaches that the fluid present at any moment in a cooled area never reaches the area to be cooled. The device of this invention teaches the opposite in that all the fluid in the cooled area will reach the area to be cooled, rotates the magnetic material in and out of the magnetic field, and the fluid is pumped through the material. Thus, rapid cycle rates are allowed in this device.
3. Utility
Enormous utility exists for the device of this invention in that the cost of a high temperature magnetic refrigerator such as described in this application is one-tenth the cost of an equivalent gas refrigerator and operates on one-fifth of the electrical power. Because high pressure compressors and gas-gas heat exchangers are not required, the magnetic refrigerator of this invention has a high degree of mechanical reliability. In addition to its use as a low temperature magnetic refrigerator, the device has further utility in that it can be used as a magnetic engine, i.e., a refrigerator run backwards. The engine would convert low grade heat, for example, reactor waste heat, geothermal heat, solar heat, and ocean heat, into electricity in a most efficient and economical manner.